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Gamma-ray burst

Brightest stellar explosion heralds new type of long-distance astronomy.Provided by the University of California, Berkeley

Published: Thursday, September 11, 2008

Comparison of the same field, with the GRB at the center, at two minutes (imaged by PAIRITEL) and two months later (imaged by Gemini-North). The burst has faded by a factor of over a million.

A flash of light that blinded even small telescopes 6 months ago was the brightest astronomical explosion ever observed — visible to the naked eye despite originating halfway across the universe.

The gamma-ray burst, catalogued as GRB 080319B, was the result of a massive star's explosion 7.5 billion years ago that sent a pencil-beam of intense light on a direct collision course for Earth. It is the only known gamma-ray burst to have had a visible component bright enough to see with the naked eye.

"This was the brightest optical and infrared event that mankind has ever recorded," said Joshua Bloom, an assistant professor of astronomy at the University of California, Berkeley, and first author of an analysis of the event submitted to The Astrophysical Journal (ApJ) less than a week after the burst and accepted this week. "When more of these events are detected, we will open up the possibility of studying the infant universe with this new tool."

The gamma-ray burst was first detected by NASA's Swift satellite on March 19, after which many Earth- and space-based telescopes slewed into position to observe the rapidly fading light. Situated within the constellation Boötes, its flash eventually was pinpointed at a distance of about 7.5 billion light-years.

Bloom's group, using a robotic telescope in Arizona, began observing the intense infrared light just 54 seconds after the event began. The telescope, called the Peters Automated Infrared Imaging Telescope (PAIRITEL), was operating autonomously on a direct link from the Swift satellite. Bloom's graduate students Daniel Perley and Adam A. Miller analyzed the data.

"This was the most powerful event ever seen in human existence," enthused Bloom's coauthor Alex Filippenko, UC Berkeley professor of astronomy. "A star that blew up and could be seen — barely — with the naked eye on a dark, moonless night, even though seven and a half billion light-years away, is just astonishing."

Filippenko calculated that if the supernova were located about 6,000 light-years from Earth, the gamma-ray burst would have appeared as bright as the Sun. Perley noted, too, that the burst at its peak was about 200 million times brighter than the entire galaxy in which it occurred.

Filippenko said that it is unlikely that anyone actually saw the flash, because it lasted only a few tens of seconds and appeared on a night with a bright Moon. Astronomers know it reached a magnitude of 5.6, close to the limits of human vision, because a Polish observing program known as Pi of the Sky took several photos of the gamma-ray burst's afterglow at the Las Campanas Observatory in Chile.

The Katzman Automatic Imaging Telescope (KAIT) at Lick Observatory also followed the fading afterglow of the burst, as did the Gemini South telescope in Chile. Bloom and his colleagues combined these observations with Swift data and Pi of the Sky images to complete their analysis.

The assimilation of new information about the event was particularly noteworthy. Less than 6 days after the event, Bloom's group submitted a 42-page paper written during a planned mountain retreat. Miller, then new to the fast-paced gamma-ray burst field said, "It was quite an introduction to research — my heart was racing for a week. I'm glad we decided to write instead of ski!"